Remote forcing of sea surface temperature (SST) variations in the Indian Ocean
during the course of El Niño-Southern Oscillation (ENSO) events is
investigated using NCEP reanalysis and general circulation model (GCM)
experiments. Three experiments are conducted to elucidate how SST variations
in the equatorial Pacific influence surface flux variations, and hence SST
variations, across the Indian Ocean. A control experiment is conducted by
prescribing observed SSTs globally for the period 1950-99. In the second
experiment, observed SSTs are prescribed only in the tropical eastern Pacific,
while climatological SSTs are used elsewhere over the global oceans. In the
third experiment, observed SSTs are prescribed in the tropical eastern Pacific,
while a variable-depth ocean mixed layer model is used at all other ocean grid
points to predict the SST.

Composites of surface fluxes and SST over the Indian Ocean are formed based on
El Niño and La Niña events during 1950-1999. The surface flux
variations in the eastern Indian Ocean in all three experiments are similar
and realistic, confirming that much of the surface flux variation during ENSO
is remotely forced from the Pacific. Furthermore, the SST anomalies in the
eastern tropical Indian Ocean are well simulated by the coupled model, which
supports the notion of an "atmospheric bridge" from the Pacific. During boreal
summer and fall, when climatological winds are southeasterly over the eastern
Indian Ocean, remotely forced anomalous easterlies act to increase the local
wind speed. SST cools in response to increased evaporative cooling, which is
partially offset by increased solar radiation associated with reduced rainfall.
During winter, the climatological winds become northwesterly and the anomalous
easterlies then act to reduce the wind speed and evaporative cooling. Together
with increased solar radiation and a shoaling mixed layer, the SST warms
rapidly. The model is less successful at reproducing the ENSO-induced SST
anomalies in the western Indian Ocean, suggesting that dynamical ocean
processes contribute to the east-west SST dipole that is often observed in
boreal fall during ENSO events.